Purpose of DC charger for secondary battery

Hi I am planning to put a second lifepo4 battery in my van. To supply fridge and inverter. In the old days I simply put a Relais and a fuse between engine battery and second battery. The Relais would switch on when the engine was running. But now I read about DC chargers like ve Orion TR. Instead of the Relais. But why is it needed. I mean the battery has it's own BMI module. Limiting the max current and so on.